feat: Keep track of a min_depth value

Should prevent weird situations where we accidentally gapfill into backfill territory

src/database/maps.rs

@@ -187,6 +187,10 @@ pub(super) static MAPS: &[Descriptor] = &[
 		val_size_hint: Some(8),
 		..descriptor::RANDOM_SMALL
 	},
+	Descriptor {
+		name: "roomid_mindepth",
+		..descriptor::RANDOM_SMALL
+	},
 	Descriptor {
 		name: "roomserverids",
 		..descriptor::RANDOM_SMALL